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Allowing for movement

Many builders nail the aluminum valley flashing along the edges to hold it in place. But metal flashing that is loaded with nails will buckle as it expands, making for an unsightly valley. In my area of northeastern Pennsylvania, for instance, a 10-foot length of aluminum coil stock will change up to 1/2 inch in length between the cold of winter and the heat of summer. To allow for this movement, I fold a hem along the edge of the valley flashing and use site-bent clips to hold the flashing in place (Figures 2 and 3).

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Figure 2. To allow aluminum flashing to expand and contract, the author uses a site-bent hem-and-clip system to secure valley flashing.

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Figure 3. He places the hem clips every 2 to 3 feet along the edge of the flashing.

To prevent the flashing from creeping downhill, I drive one nail at the top edge of the flashing. Before I position the flashing, I again clear out any loose nails that have found their way into the valley.

Laying a Closed Valley

There are three generally accepted methods for shingling a valley: the closed valley, the open valley, and the woven valley. I use the same underlying flashing details for all three methods, and any of the methods will perform satisfactorily when installed properly.

Closed valley

I prefer the closed valley. It's the quickest and easiest to install, it protects the valley with a double layer of shingles, and, to my eye, it's the best looking.

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The finished valley has a crisp appearance - and three layers of protection.

After I've installed the valley flashing, I begin shingling the main roof. When I reach the valley, I run the first layer of valley shingles all the way through the valley and onto the secondary roof plane (see illustration). The trick here is to avoid driving any nails through the aluminum. Nails driven at the edge of the flashing may not cause a leak, but they will "short circuit" the expansion clips. At some point, however, the shingle coursing will naturally end in the middle of the valley. In this case, I remove a full tab or two from the preceding shingle so that the "run-through" valley shingle falls where it can be nailed on either side of the flashing. Once the run-through shingles are in place, I shingle the secondary roof plane, trimming these shingles at the centerline of the valley. The line formed by the cut shingles is a strong visual cue, so I make sure my cuts are straight and the finished valley line is crisp. Homeowners and superintendents will quickly complain about a wavy valley.

Open valleys

In an open valley, the shingles stop short of the valley centerline, leaving the flashing exposed. I'm not sure why open valleys are used so often. Twice as many shingles must be cut at the valley, and the exposed flashing is not protected by a layer of shingles. When a customer insists on an open valley, I install the membrane and flashing as though it were a closed valley. I lay out my shingle cuts so the open portion of the valley is slightly wider at the bottom than the top. The widening trough allows ice and snow to creep out of the valley.

Woven valleys

A woven valley is essentially a closed valley formed by alternating the shingles that run through the valley. The good news is that none of the through shingles need to be cut to length. But for some reason, I'm always frustrated by shingles that creep up on the coursing lines as I weave the valley. The finished valley also has a "mushy" appearance. For these reasons, I try to avoid woven valleys. Whichever style of valley you choose, take the time to install it correctly. A leaky roof leads to a leaking wallet. >